Immobilized-cell bioreactor technology provides a cost-effective means for the treatment of existing environmental waste problems, such as contaminated groundwater, or for the eradication of pollutants as their point of origin. This technology generally involves the colonization of a specialized microorganism onto inorganic biocarriers as fixed films and the utilization of these colonized surfaces in controllable reaction vessels or bioreactors.
Ideally, suitable biocarriers for immobilization of microorganisms should be non-toxic and should provide a rough, irregular surface. The matrix should be hydrophilic (23) and porous (17). These properties have been shown by others to promote the adherence and proliferation of microorganisms. However, the porosity of a biocarrier has proven to be of particular importance in the immobilization of microorganisms, the leading factor of which having been found to be the degree of porosity and the size of the pore. In this connection, literature has taught that biocarriers suitable for immobilization of microorganism should contain a large fraction of macropores having a diameter equal or greater than the size of the microorganism.
DE-OS 28 39 580 discloses a number of porous carrier materials for immobilization of microorganism, 70% or more of the pores of which are at least as large as the smallest dimension of the microorganism, but smaller than 4 to 5 times the greatest dimension (in yeast cells or bacteria).
U.S. Pat. No. 5,096,814 discloses a porous inorganic carrier useful for immobilizing living cells (microorganism and animal cells) for the purification of waste water or the biotechnological production of nutritional essential or pharmacological substances. The porous carrier bodies were described as typically having an open pore volume of 35% to 85%, 20% to 80% being accounted for by macropores having a diameter of 20 to 500 .mu.m.
U.S. Pat. No. 5,395,808 also teaches porous inorganic carrier bodies suitable for use as supports for living cells, such as bacteria. The bodies have a significantly large average pore diameter of about 0.5 to 100 micron (i.e., 5,000 to 1,000,000 .ANG.) and a total pore volume of from about 0.1 to 1.0 cc/g.
Large pores were heretofore believed to be essential in biocarriers to adequately immobilize living cells for two reasons. First, the rate of gaseous diffusion in pore increases with increasing pore diameter. Thus, in the case where intraparticle diffusion limits the rate of catalyzed reaction, the use of a biocarrier having large pores was thought to enhance the rate of product formation.
Second, large pores in the biocarrier body allow living microorganism cells, e.g., bacteria, to be supported with the pores of the carrier body. Bacteria are typically large, with dimensions on the order of 1 .mu.m or greater, and thus was believed not to fit in smaller pores. The presence of the bacteria in the pores was heretofore believed to be necessary to promote large bacterial populations (due to additional surface area available for colonization). The greater the concentration of bacteria, i.e., living cells, the greater the catalytic activity of the immobilized biomass. Further, bacteria in the pores of the biocarrier was believed to be required to protect the microorganism from transient upsets in the external medium because of slow rates of diffusion into the pores.
Porous inorganic carrier having large pore size suffer from various disadvantages. These include difficulty in tailing pore size to a specified range, the necessity for added burnout agents to introduce the desired porosity, poor physical integrity or dimensional stability, difficulty and costs in processing and limited shapes and sizes of the bodies. Consequently, there exists a need for porous, inorganic carriers to immobilize living cells, i.e., bacteria and other microorganism, which carriers promote comparable or greater bioactivity heretofore attributed to inorganic biocarriers having a large macropore volume, while suffering none of the disadvantages associated therewith.
Accordingly, it is an object of this invention to ameliorate or eliminate the forementioned problems by providing a carrier for immobilizing living cells which offers the advantages of ease of processing, low cost production, and good physical integrity.
It is also an object of this invention to provide a method of using a porous inorganic support as a carrier for immobilizing living cells which support contains a low pore volume in the macropore range.
It is further an object of the invention to provide a method of using a porous, non-macroporous, inorganic carrier in a bioreactor as a support for living microorganisms.
Another object of the present invention is to provide a porous, non-macroporous inorganic carrier/biocatalyst composite useful in a bioreactor system.
It is also an object of the invention to provide a bioreactor system using such carrier-biocatalyst composite.
Other facets and advantages of the present invention will be apparent from the ensuing description and the appended claims.